1. Field of the Invention
This invention relates to a left/right drive torque adjusting apparatus for a vehicle and also to a left/right drive torque adjusting method for a vehicle. These apparatus and method are suitable for use in the distribution of drive torque to left and right drive wheels in a four wheel drive or two wheel drive automotive vehicle or in the distribution of drive torque by transfer of power between left and right non-drive wheel (i.e., wheels other than the drive wheels) in a two wheel drive automotive vehicle.
2. Description of the Related Art
Recent years have seen major developments in four wheel drive automotive vehicles (hereinafter called "4WD vehicles"), including a variety for full-time 4WD automotive vehicles where improvements include positive adjustment of the distribution of torque between front and rear wheels.
Taking in a broad sense a mechanism for distributing torque to left and right wheels in an automotive vehicle, on the other hand, it is considered to include conventional normal differentials as well as LSDs (limited slip differentials) including those of the electronic control type. They however do not positively adjust the distribution of torque, so that they cannot distribute torque between left and right wheels as desired.
With a view to permitting positive adjustment of the distribution of torque between left and right wheels, apparatuses have been proposed as shown in FIG. 28 and FIG. 29.
FIG. 28 is a cross-sectional view of a rear differential 122 with an electromagnetically-driven differential limiting mechanism, which is disclosed in SAE Paper No. 905078. As is illustrated there, an input shaft 401 is connected to a rear end of a drive shaft 120. A drive pinion gear 402 is supported for integral rotation with the input shaft 401. Further, the input shaft 401 is rotatably supported in a front part of a case 413 by way of a bearing 412.
A crown gear 403 is rotatable in mesh with the drive pinion gear 402, and a power-transmitting, ring-shaped member 404 and a first housing 405 are integrally connected to the crown gear 403 by bolts 431.
A rear differential 122 is a planetary gear differential making use of a planetary gear mechanism and is constructed on and in the power-transmitting, ring-shaped member 404. The rear differential comprises a ring gear 407 formed on an inner peripheral wall of the ring-shaped member 404, a sun gear 408 connected with a left-wheel axle through splines, a carrier 409 connected with a right-wheel axle through splines, and planetary gears 411a, 411b mounted on the carrier 409 via shafts 410a, 410b, respectively.
Rotational torque inputted through the input shaft 401 is transmitted through the drive pinion gear 402 and the crown gear 403 and then from the ring gear 407 of the ring-shaped member 404 to the right-wheel axle via the planetary gears 411a, 411b and the carrier 409. At the same time, it is also transmitted to the left-wheel axle via the planetary gears 411a, 411b and the sun gear 408.
A second housing 406 is disposed on a right side of the carrier 409. This second housing 406 is supported by a ring-shaped support member 418 via a bearing 428.
The rear differential 122 is provided with a differential limiting device 123, which is constructed of a multi-plate clutch 414 as a differential limiting mechanism, drive means 417 for driving the multi-plate clutch, and rear differential control unit 48A of a controller for controlling the drive means 417 via a power amplifier 48C.
The multi-plate clutch 414 is therefore disposed inside the ring-shaped member 404. A holder portion 415a which supports one group of clutch disks 414a thereon is connected to the carrier 409 via the shafts 410a, 410b so that the clutch disks 414a can rotate integrally with the carrier 409. The holder portion 415b which supports the other group of clutch disks 414b thereon is formed on a hollow axle 416 on which the sun gear 408 is mounted, whereby the clutch disk 414b is rotatable integrally with the sun gear 408.
The drive means 417 is constructed of a force direction change-over mechanism 429 interposed between the carrier 409 and the second housing 406 and an electromagnetic clutch system 430 for driving the force direction change-over mechanism 429.
The electromagnetic clutch system 430 in turn comprises a clutch 427 interposed between a ring-shaped member 423 and a member 426 located on a side of the second housing 406, a magnet 419, and a solenoid (EMCD coil) 420 as the control means for the differential limiting mechanism.
Based on detection information from individual sensors (wheel speed sensor, steering angle sensor, side acceleration sensor, longitudinal acceleration sensor, throttle position sensor, engine rpm sensor, shift position sensor, etc.), the rear differential control unit 48A sets target clutch torque for the multi-plate clutch 414 and then controls a current to be fed to the electromagnetic clutch system 430 of the drive means 417 so that the target clutch torque can be obtained.
FIG. 29, on the other hand, is a cross-sectional view illustrating a rear differential 522 disclosed in Japanese Patent Application Laid-Open No. HEI 4-232127, which is equipped with another differential limiting mechanism. In FIG. 29, reference numerals similar to those employed in FIG. 28 identify substantially like parts. A differential limiting mechanism 523 of the rear differential 522 is constructed of a multi-plate clutch 414, drive means 517 for driving the multi-plate clutch 414, and a rear differential control unit 48A of a controller for controlling the drive device 517. The rear differential 522 is of the bevel gear type and is constructed of an input pinion 522A and left and right, driven bevel gears 522B, 522C.
The drive means 517 uses pneumatic pressure. A pneumatic circuit 417A to which a pump 417B is connected is controlled by the rear differential control unit 48A, whereby an air piston 417D is driven via an air feed line 417C to regulate the engagement pressure for the multi-plate clutch 414.
Based on detection information from individual sensors 48B, the rear differential control unit 48A sets target clutch torque for the multi-plate clutch 414 and then controls the pneumatic pressure circuit 417A of the drive means 517 so that the target clutch torque can be obtained.
In parallel with torque distribution adjusting apparatuses between front and rear wheels, development of an apparatus capable of controlling the distribution of torque between left and right wheels is also desired. In this case, the adjustment is directed not only to the distribution of torque between left and right drive wheels in a 4WD vehicle but also to the distribution of torque between left and right drive wheels in a 2WD vehicle.
If the distribution of torque is taken in such broad sense that it covers not only the distribution of torque outputted from an engine but also the transmission of torque which takes place through transfer of drive torque between left and right axles, it can be contemplated to adjust the distribution of torque between left and right non-drive wheels (as opposed to drive wheels) in a 2WD vehicle.
Neither the left non-drive wheel nor the right non-drive wheel receives drive torque from the engine. If it is however possible to realize transfer of drive torque from one of these non-drive wheels to the other, brake force can be developed on the side of the former non-drive wheel and drive torque can be produced on the side of the latter non-drive wheel. Accordingly, it becomes possible to adjust the distribution of torque (including negative drive torque, namely, brake force) between the left and right non-drive wheels.
Further, desired as such a left/right drive torque distribution adjusting apparatus for a vehicle is one that can perform the distribution of torque without inducing any large torque loss or energy loss. Such a drive torque distribution adjusting apparatus is also desired to permit a dimensional reduction.
As a drive torque transmission control apparatus capable of adjusting drive torques for the left-wheel and right-wheel axles by transferring drive torque between the left-wheel and right-wheel axles, it seems to be feasible to employ such a construction that a rotational speed on a side of one of the left-wheel and right-wheel axles is increased or decreased and outputted at a predetermined shift ratio and an output means for the drive torque so increased or decreased and a side of the other axle are coupled together, for example, by a slippable coupling such as a slip clutch to transfer drive torque from the side rotating at a higher speed to the side rotating at a lower speed.
When a rotational speed on the side of one of the left-wheel and right-wheel axles is increased and outputted at a predetermined shift ratio, for example, the output means for the rotational speed so increased rotates at a higher speed than a rotational speed on the side of the other axle. Coupling of the output means with the other axle therefore makes it possible to transmit drive torque from the side of the output means, which is on the side rotating at the higher speed (i.e., the side of the one axle), to the side rotating the lower speed (i.e., the side of the other axle). Accordingly, arrangement of such torque transmission mechanisms on the left and right sides, respectively, permits transmission of drive torque from the side of the left wheel to the side of the right wheel and vice versa as desired.
When a rotational speed on the side of one of the left-wheel and right-wheel axles is decreased and outputted at a predetermined shift ratio, on the other hand, the output means for the rotational speed so decreased rotates at a lower speed than the side of the other axle during normal running where the difference in rotational speed between the left wheel and the right wheel is small. Coupling of the output means with the other axle therefore results in transmission of drive torque from the side rotating at the higher speed (i.e., the side of other axle) to the side rotating at the lower speed, that is, the side of the output means (i.e., the side of the one axle). Here again, arrangement of such torque transmission mechanisms on the left and right sides, respectively, permits transmission of drive torque from the side of the left wheel to the side of the right wheel and vice versa as desired.
Upon turning of a vehicle, however, a difference in rotational speed inevitably occurs between each inner wheel and its corresponding outer wheel so that the rotational speed becomes higher on the side of the outer wheel. If the ratio of a rotational speed of the inner wheel to that of the outer wheel is great during turning, output of the rotational speed on the side of the inner-wheel axle, for example, at an upshift ratio therefore does not necessarily make output means for the thus-increased rotational speed rotate at a higher speed than the outer-wheel axle. Accordingly, each of the left and right torque transmission mechanisms can perform transmission of drive torque from the side of the outer wheel to the side of the inner wheel but, occasionally, may not perform transmission of drive torque from the side of the inner wheel to the side of the outer wheel.
A discussion will now be made of a situation where a vehicle makes a turn (a right turn in this instance) about a center C as shown in FIG. 30. Representing the distance (tread) between a left wheel W1 and a right wheel Wr of the vehicle by Lt, the turning radius by R, the speed of the turning outer wheel by Vo, the speed of the turning inner wheel by Vi, the wheel base by L, the actual steering angle by .delta., and the stability factor by A, the wheel speed difference .DELTA.Vhr due to the difference in radius between the track of the turning inner wheel and that of the turning outer wheel can be derived as will be described next, provided that a vehicle body slip angle .beta. is sufficiently small. From cos .beta..apprxeq.1, sin .beta..apprxeq..beta.y, EQU .DELTA.Vhr=Vo-Vi=(Lt/R).multidot.V
where EQU R=(1+A.multidot.V.sup.2)L/.delta.
Accordingly, a difference in rotational speed occurs between the left wheel and the right wheel.
Describing further on the basis of a specific example, FIG. 31 is a velocity diagram of various parts during turning of a vehicle in which the above-described vehicular left/right drive torque adjusting apparatus is arranged at a portion where a driving input Ti inputted to a case (differential case) DC of a differential from an engine is distributed to a side of a left-wheel axle S1l and a side of a right-wheel axle S1r via the differential.
On the side of the left-wheel axle S1l, the driving input Ti is increased at a predetermined shift ratio and the so-increased input is outputted to the side of a member (output means) S2l, and a coupling Tc2 is interposed between the member S2l and the differential case DC to adjust the state of coupling therebetween. On the side of the right-wheel axle S1r, on the other hand, the driving input Ti is increased at a predetermined shift ratio and the so-increased input is outputted to the side of a member (output means) S2r, and a coupling Tc1 is interposed between the member S2r and the differential case DC to adjust the state of coupling therebetween.
Assuming now that the vehicle is making a left turn, the left and right wheels then become the inner and outer wheels, respectively. Where the turning radius is small, there arises a situation under which, as is illustrated in FIG. 31, the rotational speed on the side of the left-wheel axle S1l and that on the side of the right-wheel axle S1r differ substantially from each other. Although the rotational speed of the output means S2l on the side of the left wheel is supposed to be higher than that of the differential case DC basically, that is, unless the difference in rotational speed between the left and right wheels is large, the above situation leads to the situation that the rotational speed of the output means S2l on the side of the left wheel conversely becomes lower than that of the differential case DC.
To improve the turning performance upon initiation of turning, for example, a moment can be produced in the turning direction on the vehicle by distributing more drive torque to the side of the right wheel as the outer wheel during the turning and hence imbalancing the drive torque between the left and right wheels. This is however not feasible under the above situation. To the contrary, the transfer (transmission) of the drive torque is effected in the direction opposite to the intended direction so that moment is produced in a direction opposite to the required turning direction. The above control therefore has the potential problem that the running performance is deteriorated conversely.
It may of course be possible to always permit transfer of torque in a desired direction by setting a shift ratio (the inverse of an upshift ratio or downshift ratio) at a sufficiently large value. Such a large upshift ratio however leads to greater torque transmission loss and energy loss, thus resulting in the development of such inconvenience that the gas mileage is deteriorated and the durability of elements of devices such as the multi-plate clutch is reduced due to production of heat in an increased quantity. Accordingly, it is not practical to set the shift ratio (the inverse of the upshift ratio or downshift ratio) at a sufficiently large value.
It is therefore desired to avoid transfer (transmission) of such drive torque in a direction opposite to the intended direction without development of accompanying inconvenience, for example, by ingeniously designing the control of operation of the coupling.
Upon turning of the vehicle, however, the left and right torque transmission mechanisms can each perform transmission of drive torque from the side of the outer wheel to the side of the inner wheel. It is therefore desired to adjust or control such drive torque.
In the above case, it is also desired to conduct the control of the drive torque while minimizing torque loss as much as possible.
In the control described above, the slip clutches and the like make use of the principle that torque is transmitted from a side of a higher speed to a side of a lower speed. At the same time, each slip clutch or the like also acts to decrease the velocity of slipping between members which are rotating relative to each other. Accordingly, the slip ratio of the tire of the wheel on the side to which more torque is desired to be distributed increases and, at the same time, the slip ratio of the tire of the wheel on the side to which less torque is desired to be distributed decreases. On the side of the slip ratio so increased, drive torque is increased. On the side of the slip ratio so decreased, drive torque is decreased.
To theoretically perform such control of torque distribution, a tire characteristic must be linear as a pre-requisite. The tire characteristic can be indicated by means of a tire characteristic diagram which diagrammatically illustrates a relationship (.mu.-S characteristic) between the slip ratio S (%) and the road surface drive-limiting friction coefficient .mu., for example, as shown in FIG. 32.
The friction coefficient .mu. corresponds to reaction force of the road surface to the tire. Different slip ratios S are used depending on whether the tire is in a driven state or in a braked state. When the tire is in a driven state, the slip ratio S is defined as follows: S=(V-VB)/V. When the tire is in a braked state, on other hand, the slip ratio S is defined as follows: S=(VB-V/VB.
As is depicted in FIG. 32, the friction coefficient .mu. linearly increases with the slip ratio S in a region where the slip ratio S is small (for example, a region in which the slip ratio is about 20% or smaller), so that the friction coefficient .mu. is in a linear region. In a region in which the slip ratio S is large (for example, a region in which the slip ratio is greater than about 20%), on the other hand, the friction coefficient .mu. decreases as the slip ratio S increases, so that the friction coefficient .mu. is in a non-linear region.
In the non-linear region, restraining force by the slip clutch or the like supersedes road-surface gripping force of the tire so that the slip clutch or the like is eventually locked up, leading to the problem that the above-described theoretical control of characteristic becomes no longer applicable.